На обеих сторонах
не дал он нам такое закоммитить
это имхо шляпа какая - то(но это не точно)
вот тут есть описание как оно работает
MAC address synchronization enables an MC-LAG peer to forward Layer 3 packets arriving on MC-AE interfaces with either its own IRB MAC address or its peer’s IRB MAC address. Each MC-LAG peer installs its own IRB MAC address as well as the peer’s IRB MAC address in the hardware. Each MC-LAG peer treats the packet as if it were its own packet. If MAC address synchronization is not enabled, the IRB MAC address is installed on the MC-LAG peer as if it was learned on the ICL.
https://www.juniper.net/documentation/en_US/release-independent/nce/topics/concept/mc-lag-on-core-understanding.html#jd0e487